Underground storage tanks, which are typically made of fiberglass reinforced plastic, are well known in the art. Such tanks are commonly used to store gasolines and other fuels, as well as corrosive liquids. Typically, these tanks have a plurality of spaced-apart ribs around the circumference of the generally cylindrical tank. Recent innovations in such underground tanks include the development of a double wall tank. Double wall tanks provide a double barrier to prevent leakage of the liquids from the tank. Also, the annular space between the inner and outer walls can be filled with a leak detecting fluid which can be monitored to detect leaks in the tank's inner or outer wall. As disclosed in U.S. Pat. No. 4,676,093 to Pugnale et al., the annular space between the inner and outer tank walls can be connected to the hollow ribs in order to form a leak detecting space between the inner and outer walls.
One of the problems with previous designs for double wall tanks is that the outer layer is spaced apart a considerable distance from the inner layer, thereby creating a large volume in the void or annular space between the outer and inner walls. This space is generally defined by the height of the ribs, since the double wall tank is usually made by superimposing an outer wall over the ribs of a ribbed single wall tank. Since the void space or annular space is so large, a large volume of leak detecting fluid is required in order to operate the tank leak detection system. This leak detection fluid is expensive and it would be desirable to provide a tank having a smaller volume in the annular space between the inner and outer tank walls.
Another problem caused by the fact that the outer tank wall of previous designs is spaced apart from the inner tank wall by the height of the rib, is the fact that the resulting double wall tank is considerably larger in gross exterior volume than previously employed single wall tanks. These large double wall tanks, when installed in a replacement operation, such as in a gasoline service station, result in the need for a larger excavation for installation of the new double wall tank. The larger excavation results in a considerable amount of excess soil which must be removed from the excavation and disposed of. Such removed soil is considered "contaminated" by most environmental regulations, and disposal of such excess soil is expensive.
It would be desirable to provide a double wall tank in which the void space or annular space is reduced from that of previous designs, and in which the outer wall dimension does not greatly exceed the dimension of existing single wall tanks.